Authorities say the child was wandering unattended just after midnight.
Responding officers learned that the child's mother, 36-year-old Michelle Burns, lived in the complex, and the child's aunt reported that her sister was not answering her phone.
She also reported that she heard arguing coming from inside the apartment, where Burns resided with her boyfriend, Jeffrey Daniels, earlier in the evening.
She said Daniels had knocked on her door while carrying two small children and had asked for a ride.
Officers entered Burns' apartment, where they discovered her dead with stab wounds to the neck.
Soon thereafter, police in next town over located the 36-year-old Daniels walking with the victim's two other children, ages 18 months and 6 months.
He was taken into custody and is charged with murder, possession of a weapon for unlawful purposes, and endangering the welfare of a child.
WATCH: Aerial footage from WABC of the apartment complex
According to a neighbor, there was constant friction between the victim and her boyfriend.
Police said there were no signs the victim put up a fight.
The 5-year-old child, from a previous relationship, was apparently left in the apartment with the victim, while the suspect left with the other two.